Some modern hydraulic systems which are supplied by variable displacement pumps are of the type referred to as "load responsive". A load responsive system permits the variable displacement pump to pump only that flow and pressure necessary to move a particular load and is typified in U.S. Pat. No. 3,401,521. The speed which the load is moved can be controlled by the spool position of the directional control valve in the system which senses a pressure drop across the valve spool and accordingly adjusts the pump output as typified in U.S. Pat. No. 4,037,410.
The above-mentioned prior art "load responsive" systems sense the pressure drop across the incoming flow from the pump to the motor.
The present invention senses the pressure drop on the outlet side of the load, which is also the return flow from the motor, rather than the inlet side as done in the lastmentioned patent. It is advantageous to meter on the outlet side of a load in various situations such as craning of a heavy gravity load. Outlet metering also provides inherent cavitation control up to the flow capacity of the pump. Outlet metering also allows an added system stiffness, by maintaining a slight pressure on the low pressure side of any gravity load as it is being lowered or stopped. The logic required for a meter-out system is different than the conventional meter-in type system of the prior art material above. However, the meter-out system can be used with the same pressure and flow compensators descried in the meter-in system of U.S. Pat. No. 3,508,847.